This invention is concerned with a form of photosensitively colored glass known as polychromatic glass. It is particularly concerned with a new and improved method of selectively developing a multi-colored image in such a glass.
Polychromatic glasses are capable of having a range of colors developed therein by certain combinations of radiation exposure and thermal treatment. Colors in such glasses result from a unique combination of alkali fluoride.multidot.silver halide microcrystals and metallic silver particles. It is not certain whether the silver is coated on, or contained in, the microcrystals, or whether it exists separately therefrom in minute discrete particles. Variation in color results from variation in exposure flux which is defined as the product of radiation intensity and exposure time.
The physical and chemical characteristics of polychromatic glasses, as well as general methods of development of a range of colors therein, are disclosed in detail in U.S. Pat. No. 4,017,318, granted Apr. 12, 1977 to J. E. Pierson and S. D. Stookey. U.S. Pat. No. 4,057,408, granted Nov. 8, 1977 to the same inventors, contains essentially the same disclosure and claims method aspects of the invention. The entire teaching of these patents is incorporated herein by reference to avoid repetition.
The patents generally disclose that polychromatic glasses may have widely-varying compositions, but must contain silver, alkali oxide, fluoride, at least one halide selected from the group of chloride, bromide and iodide, and, if ultra-violet radiation is employed, cerium oxide in minute amount. They further describe a general method of color production comprising four distinct steps:
(1) exposure to actinic or high energy radiation, PA1 (2) heat treatment at a temperature between the glass transformation range and softening point, PA1 (3) re-exposure of the glass to actinic or high energy radiation, and PA1 (4) reheating to a temperature within or above the transformation range, but below the glass softening point.
In accordance with the patent teachings, a range of colors may be developed in a glass of suitable composition by first exposing the glass to actinic (ultra-violet) radiation in the range of 2800-3500 A, or to high energy radiation such as X-rays or high velocity electrons. The amount of exposure flux from this exposure determines the depth and ultimate hue of the color to be produced in the glass, although only a latent image is developed in the glass during this initial exposure.
The glass is then subjected to a heat treatment at a temperature ranging from about the transformation range of the glass up to approximately its softening point. The glass composition selected, as well as the temperature and duration of the heat treatment, determine whether the final product will be transparent or opacified. The heat treatment causes colloidal silver particles to precipitate and serve as nuclei for the growth of alkali fluoride.multidot.silver halide microcrystals.
The nucleated glass is then exposed to high energy or actinic radiation a second time. The intensity and time of this second exposure depends on the desired intensity or saturation of the hue which was predetermined by the first exposure.
Finally, the glass is heated to a temperature within, or somewhat above, the transformation range, but below the glass softening point. It is believed that submicroscopic particles of metallic silver precipitate on the alkali fluoride.multidot.silver halide crystals during this heat treatment, thereby giving rise to color in the glass. The transformation range of a glass is commonly considered to lie in the vicinity of the glass annealing point.
U.S. Pat. No. 4,092,139, granted May 30, 1978 to J. Ference, discloses an alternate color developing procedure whereby the second exposure (the re-exposure) of the Pierson-Stookey method is carried out at a temperature between 200.degree. and 410.degree. C. to cause the metallic silver to precipitate, thus obviating the separate reheating step. Also, U.S. Pat. No. 4,134,747, granted Jan. 16, 1979 to Pierson and Stookey, describes a procedure wherein the potentially polychromatic glass is melted in a reducing atmosphere. During the color development procedure, this modification produces a so-called reverse opal effect, whereby exposed portions of the glass remain transparent, although colored, and unexposed portions of the glass become opacified.